1. Field of the Invention
The invention relates to a method for detecting ionisable gases, in particular organic molecules, preferably hydrocarbons. The invention also relates to an apparatus for detecting ionisable gases, includins a gas inlet, means for ionising an ionisable gas, including a voltage source, two electrodes and means for determining a current strength, the two electrodes being connected to the voltage source and the means for determining a current strength being connected to the electrodes such that the strength of the current flowing between the electrodes can be measured.
2. Description of Related Art
In order to detect organic molecules, for example hydrocarbons in a gas mixture, the flame ionisation detection method is frequently used. In such a method, the gas mixture to be examined is introduced into a flame, in particular into an oxyhydrogen flame. In this flame, the ionisable components of the gas mixture, i.e. in particular organic molecules, are ionised by the thermal energy. The free electrons and ions generated in this way, for example CHO+, will then move according to their charge to respectively one of two electrodes provided, between which a voltage is applied. The current flowing between the electrodes in this way is a measure for the proportion of the ionisable gas, i.e. the concentration of the organic molecules, in the overall gas mixture.
The flame ionisation detection method has the disadvantage that hydrogen and oxygen have to be provided in order to generate the flame. Further, the environment of the burner is intensely heated by the flame. A further disadvantage is that part of the organic molecules is oxidised by the flame. These will then no longer contribute to the current flowing between the electrodes and will therefore not be detected. Therefore, this method is inaccurate.
The concentration of organic molecules in a gas mixture has to be determined, inter alia, in the case of exhaust gases. Thus, for example in the case of exhaust systems, legal requirements in respect of maximum concentrations of certain organic substances have to be complied with. In order to meet such standards, controlled exhaust gas purification systems are often used in the prior art. The measured variable of the concentration of the organic molecules in exhaust gas, which variable is required for controlling, is obtained here, for example, by means of a flame ionisation detector. However, these have the disadvantage that a combustible gas such as for example oxyhydrogen gas is required for the operation of these detectors. Depending on the position of the detector in the exhaust system, however, it is complex, dangerous or even impossible to ensure a supply with this combustible gas.
Determining the concentration of organic molecules in a gas mixture is also required in the general area of gas analytics, for example when a process gas is to be analysed. The disadvantage of the use of flame ionisation detectors here is that the process gas is contaminated by any incompletely burnt proportions of the combustible gas used to feed the flame or by combustion products. This may have a negative effect on the process steps following the analysis.